A flame scanner monitors the combustion process in a fossil fuel fired combustion chamber to provide a signal indicating the presence or absence of a stable flame. With the presence of a stable flame, fossil fuel continues to be fed into the combustion chamber of the steam generator. In the event that the flame becomes unstable, or the flame is lost completely (known as a flame out condition), the flame scanner provides a loss of flame signal. Based upon a loss of flame signal, fossil fuel delivery to the combustion chamber can be discontinued before an undesirable unstable operating condition or flame out condition develops. In some systems, a human operator interrupts the fuel supply based upon the loss of flame signal; in other systems a burner management system (BMS) interrupts the fuel supply based upon the loss of flame signal.
Conventional flame scanners produce an electrical signal based upon a monitored flame. This resulting analog electrical signal is transmitted to processing electronics that are housed separately from the flame scanner, typically in an equipment rack located adjacent to a control room. The strength of the produced signal is typically proportional to the intensity of the monitored flame. If the signal strength falls below a lower set point, or rises above an upper set point, delivery of main fuel into the combustion chamber is interrupted. Set points are sometimes referred to as trip points.
One type of flame scanner is an ultraviolet tube flame scanner which produces a pulsed electrical output whose pulse rate is proportional to the intensity of ultraviolet light, in the range of approximately 250 to 400 nanometers, emitted by a flame. These scanners are particularly suited for monitoring gas flames since the emission from gas flames can be primarily in the ultraviolet range, with only minimal visible light emissions. Ultraviolet flame scanners based on Geiger mueller tubes require extensive maintenance and have relatively limited operational lives as well as unstable failure modes.
Another type of flame scanner is a photodiode flame scanner. Photodiode flame scanners are the most prevalent type of flame scanner in use today in industrial application. In these flame scanners, visible light, in the range of approximately 400 to 700 nanometers, is collected from inside a combustion chamber, transmitted through a fiber optic cable, and directed onto a single photodiode to produce an electrical signal utilized by the separate processing electronics. Photodiode flame scanners are well suited for monitoring oil and coal flames, as emissions from such flames are in the visible and near infrared ranges.
Photodiode flame scanners mount on utility or industrial boilers and include two primary components. One component is a removable flame scanner assembly, i.e., a flame sensor and fiber optic cable. The flame sensor senses energy from the boiler via light transmission from the boiler flames by way of the fiber optic cable. The other component of the flame scanner includes a scanner guide pipe, which is a fixed, structural part of the boiler and disposed within the combustion chamber of the boiler. The flame scanner assembly fits into the guide pipe. In order for maximum efficiency of light transmission from the flame front inside the boiler to the flame sensing electronics located outside of the boiler, the tip of the flame scanner assembly must be seated firmly at a corresponding fireside end of the guide pipe. Therefore a length of the removable flame scanner assembly must match a length of the scanner guide pipe within fractions of an inch. Preferably, the flame scanner assembly is manufactured to be ⅜″ to ½″ longer than the guide pipe to insure compression of the flame scanner assembly to seat the tip of the flame scanner assembly firmly at the fireside end of the guide pipe.
There has been a long history of flame scanner assembly/guide pipe dimensional size issues, i.e., variation in length when installing and mating the two primary components. For example, referring to FIGS. 2 and 3 only for reference to dimensions of the longitudinal length of the flame scanner assembly and guide pipe, respectively, some of the design and fit-up issues of a flame scanner include matching an “A” dimension of the guide pipe with an “L” dimension of the flame scanner assembly, where “A” is the internal length of the guide pipe for receiving the flame scanner assembly and “L” is the length of the flame scanner assembly that is disposed within the guide pipe. For example, on new orders and existing orders, mismatches between the “A” and “L” dimensions occur due to drawing revisions not being up to date or field changes to equipment not being recorded. Achieving a ½″ compression at “0” tilt on the scanner in the field has been a tremendous problem, as guide pipes tend to be installed and fit-up differently at each site. With some flame scanner assemblies, costs are incurred with the selection of variable lengths of the fiber optic cables and lengths of adaptation pipe extensions.
In addition, flame scanners often experience what is known as “pull back” during operation of the boiler (tilting) caused by guide pipes that have stretched over time. Further, guide pipes tend to sag over time. When a scanner has “pull back” issues during tilting or with old equipment, the flame scanner performance degrades substantially. Moreover, purge air is no longer directed across a lens barrel of the flame scanner assembly to remove contaminants from the lens or the quartz window when the scanner guide pipe sags or experiences “pull back”, thus reducing flame scanner performance.
Accordingly, a need exists for an adjustable/variable length flame scanner that will permit quick and easy adjustment of miss matched lengths of the guide pipe and flame scanner assembly.